Torque sensor and electric power steering system having same

ABSTRACT

A torque sensor has a torsion bar coaxially in alignment with input and output shafts, a ring shaped magnet fixed to an axial end of the input shaft, a pair of magnetic yokes fixed to an axial end of the output shaft, and a magnetic sensor for detecting magnetic flux density generated between the pair of magnetic yokes. Each of the magnetic yokes is provided with claws, which are circumferentially spaced at constant intervals, and whose number is equal to that of each of N and S poles alternately arranged circumferentially in the magnet. Each center of the claws coincides with a boundary between immediately adjacent N and S poles of the magnet, when the torsion bar is not twisted. The magnetic sensor is inserted into an axial gap between the pair of magnetic yokes without contacting the magnetic yokes.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of priorityof Japanese Patent Applications No. 2001-148894 filed on May 18, 2001,No. 2001-259961 filed on Aug. 29, 2001, No. 2001-316435 filed on Oct.15, 2001 and No. 2001-316788 filed on Oct. 15, 2001, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a torque sensor for detectingtorque applied to a torsion bar to be used in a rotating forcetransmission system, in particular, in an electric power steeringsystem.

[0004] 2. Description of Related Art

[0005] Conventionally, according to a device disclosed in JP-A-8-159887for detecting torsion torque applied to a torsion bar, a magnet and amagnetic sensor are used. The magnet is fixed to an axial end of thetorsion bar and the magnetic sensor is fixed to the other axial end ofthe torsion bar. When the torsion torque is applied to the oppositeaxial ends of the torsion bar, the torsion bar is twisted so that arotation displacement of the magnetic sensor relative to the magnet ischanged. Accordingly, an output responsive to the applied torque isgenerated from the magnetic sensor.

[0006] According to the detecting device mentioned above, electriccontacts such as a brush and a slip ring for supplying electric power toand picking up a signal from the magnetic sensor are necessary, sincethe magnet and the magnetic sensor are fixed to the opposite axial endsof the torsion bar that is rotated. The use of the brush and the slipring is prone to deteriorate reliability of the detecting device.

[0007] Further, according to another detecting device disclosed inJP-A-6-281513, though it is similar to JP-A-8-159887 in view that themagnet and the magnetic sensor are used, helical gears, to which themagnet is fixed, are used for converting the rotation displacement ofthe axial end of the torsion bar relative to the other axial end of thetorsion bar into an axial displacement of the magnet relative to themagnetic sensor that is fixed to a housing. Accordingly, the electriccontacts for supplying electric power to and picking up a signal fromthe magnetic sensor are not necessary.

[0008] However, this detecting device uses the gears so thatconstruction of the detecting device is complicated. Further, the devicehas a drawback in performance since detection errors and response delaysseem to be unavoidable due to backrush of the gears and possible wear ofthe gears.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a torque sensorwithout using electric contacts, whose construction is more compact andwhose performance is more accurate.

[0010] Another object of the present invention is to provide an electricpower steering system incorporating the torque sensor.

[0011] It is an aspect of the present invention to provide a method ofeasily assembling a ferromagnetic member to soft magnetic members in thetorque sensor.

[0012] To achieve any of the above objects, in a torque sensor fordetecting torsion torque to be applied to a first shaft and a secondshaft, a resilient member is disposed between and fixed to the first andsecond shafts so that the first shaft, the resilient member and thesecond shaft are coaxially in alignment with one another. The resilientmember being resiliently twisted, when torsion torque is applied to thefirst shaft and the second shaft. A ferromagnetic member is connected toone of a given position of the first shaft and a given position of theresilient member on a side of the first shaft and rotatable togethertherewith. The ferromagnetic member produces a magnetic field. A softmagnetic member is connected to one of a given position of the secondshaft and another given position of the resilient member on a side ofthe second shaft and rotatable together therewith. The soft magneticmember is positioned within the magnetic field and forms a magneticcircuit so that magnetic flux density generated in the magnetic circuitis varied when rotation displacement of the soft magnetic memberrelative to the ferromagnetic member is changed according to the twistof the resilient member. A magnetic sensor is positioned in a vicinityof and without contacting the soft magnetic member for detecting themagnetic flux density generated in the magnetic circuit.

[0013] With the torque sensor mentioned above, the magnetic sensor doesnot detect directly the magnetic flux generated from the ferromagneticmember. Accordingly, the magnetic sensor can be fixed, for example, to ahousing where the torque sensor is accommodated without the electriccontacts so that reliability of the torque sensor is higher.

[0014] It is preferable to have an auxiliary soft magnetic member havinga magnetic flux collective portion in the torque sensor. The auxiliarysoft magnetic member is positioned in a vicinity of the soft magneticmember for introducing magnetic flux from the soft magnetic member andconcentrating the same to the magnetic flux collective portion.Accordingly, the magnetic sensor detects the magnetic flux densitygenerated in the magnetic circuit through the magnetic flux collectiveportion. As the magnetic flux generated in the auxiliary soft materialmember is concentrated to the magnetic flux collective portion, themagnetic sensor can detect average of the magnetic flux densitygenerated over an entire circumference of the soft magnetic member.Accordingly, detecting errors are hardly caused by manufacturing errors,assembly inaccuracy of the components constituting the magnetic circuitor misalignment between the first and second shafts.

[0015] Preferably, the ferromagnetic member is a ring shaped magnethaving N and S poles alternately arranged circumferentially and the softmagnetic member is a pair of ring shaped magnetic yokes that arepositioned around an outer circumference of the magnet and axiallyopposed to each other with an axial gap therebetween. Each of themagnetic yokes has claws which are radially spaced at constant intervalsand whose number is equal to that of each of the N and S poles. Further,the claws of one of the magnetic yokes axially extend toward and arepositioned to alternate circumferentially with those of the other of themagnetic yoke. The magnetic sensor is positioned in the axial gapbetween the pair of the magnetic yokes.

[0016] With the construction mentioned above, when the angular positionof the magnet relative to the magnetic yokes when the resilient memberis twisted, the claws of one of the magnetic yokes come closer to the Nor S poles and the claws of the other of the magnetic yokes come closerto the S or N poles. Polarity of magnetic flux flowing in the one of themagnetic yokes is opposite to that in the other of the magnetic yokes.Passive or negative magnetic flux density, which is substantiallyproportional to a twist amount of the resilient member, is generatedbetween both the magnetic yokes.

[0017] Further, it is preferable that the auxiliary soft magnetic memberis a pair of ring shaped auxiliary magnetic yokes each having themagnetic flux collective portion. One of the auxiliary magnetic yoke ispositioned around an outer circumference of the one of the magnetic yokeand the other of the auxiliary magnetic yoke is positioned around anouter circumference of the other of the magnetic yoke so that themagnetic flux collective portions of the pair of the auxiliary magneticyokes are axially opposed to each other with an axial gap therebetween.In this case, the magnetic sensor is positioned in the axial gap betweenthe magnetic flux collective portions.

[0018] Furthermore, it is preferable that a length of the axial gapbetween both the magnetic flux collective portions is shorter than thatbetween both portions of the pair of auxiliary magnetic yokes other thanthe magnetic flux collective portions. This construction serves toimprove detection accuracy of the torque sensor.

[0019] As an alternative, the torque sensor may have a first rotationtransmitting member through which the magnet is connected to the one ofthe given position of the first shaft and the given position of theresilient member on a side of the first shaft and a second rotationtransmitting member through which the soft magnetic member is connectedto the other of the given position of the second shaft and the givenposition of the resilient member on a side of the second shaft. In thiscase, the magnet and the pair of magnetic yokes are positioned axiallyin parallel with the resilient member.

[0020] Preferably, the first rotation transmitting member is a firstgear fixed to the first shaft and a second gear fixed to the magnet, thefirst and second gears being in mesh with each other, and the secondrotation transmitting member is a third gear fixed to the second shaftand a fourth gear fixed to the magnetic yokes, the third and fourthgears are being in mesh with each other.

[0021] With this construction, a sensing portion such as theferromagnetic member, soft magnetic member and the magnetic sensor canbe assembled separately from the first and second shafts and theresilient member. Accordingly, it is simpler to assemble the sensingportion, for example, to the electric power steering system. Further,the sensing portion can be replaced as a single body, which facilitatesmaintenance operation.

[0022] Further preferably, each axial center of the claws of both themagnetic yokes is positioned to substantially coincide with a boundarybetween immediately adjacent N and S poles of the magnet, when a twistangle of the resilient member shows a reference value. When theresilient member is not twisted, that is, when the torsion torque is notapplied to the first and second shafts, if the axial center of the clawsis set to substantially coincide with a boundary between immediatelyadjacent N and S poles of the magnet, the torque sensor is lessinfluenced by magnetization whose value is lowered due to temperaturechange.

[0023] If two magnet sensors whose magnetism detecting directions areopposite to each other are used and, preferably are positionedsymmetrically with respect to an axis of the soft magnetic member,difference of the outputs between the two sensors can be used to canceltemperature drift of the magnet, the magnetic yokes and the magneticsensor and the sensibility of the torque sensor is doubled.

[0024] As an alternative, the magnet sensor may be more than two sensorswhich are positioned circumferentially at constant intervals and whosemagnetism detecting directions are same to one another. If the outputsfrom the sensors are processed through adding or average calculation,the detecting accuracy of the torque sensor is remarkably improvedwithout since dimensional fluctuation of magnetic circuit componentssuch as the magnet and the magnetic yokes and position fluctuation ofthe magnetic sensors are less influenced.

[0025] It is preferable that a magnetic seal covers at least outercircumference of the magnetic sensor. The magnetic seal serves toeliminate influences of terrestrial magnetism and magnetic fieldsgenerated around the torque sensor so that the erroneous detection isavoided. The magnetic seal may cover only an outer circumference of themagnetic sensor or an entire portion of the magnetic circuit of thetorque sensor.

[0026] Preferably, axial length of the magnet is longer than that of themagnetic yoke. Iron filings can be stuck to edges of the magnet withoutentering into a radial gap between the magnet and the magnetic yokes,which does not adversely affect on the magnetic circuit for detectingthe torque so that the erroneous detection may be avoided.

[0027] In case that the torque sensor mentioned above is incorporatedinto an electric power steering system for steering a vehicle wheel, oneof the first and second shafts is connected to one end of the steeringto which steering torque is applied, the other of the first and secondshafts is connected to a steering power transmission mechanism and anelectric motor gives a drive force to the steering power transmissionmechanism in response to a control current from a control circuit inresponse to a detected output of the magnetic sensor for assisting thesteering torque applied to the steering.

[0028] If the magnetic sensor is hole IC, the torque sensor is compactand inexpensive since auxiliary circuits such as a gain adjustingcircuit, an offset adjusting circuit and a temperature compensationcircuit are not necessary, so the torque sensor can be composed of lessnumber of components. Further, since the hole IC does not require anoscillating circuit so that noises are hardly radiated, the hole IC doesnot give a noise problem to surrounding electric devices.

[0029] It is preferable that the control circuit has a plate board onwhich the magnetic sensor is simultaneously mounted. In this case, wireharnesses and connectors for connecting the torque sensor and thecontrol circuit are not necessary, which results in cost saving andbetter reliability because of no electric contacts.

BRIEF DESCRIPTION OF THE DRAWING

[0030] Other features and advantages of the present invention will beappreciated, as well as methods of operation and the function of therelated parts, from a study of the following detailed description, theappended claims, and the drawings, all of which form a part of thisapplication. In the drawings:

[0031]FIG. 1 is an exploded perspective view of a torque sensoraccording to a first embodiment of the present invention;

[0032]FIG. 2A is a cross sectional view of the torque sensor of FIG. 1;

[0033]FIG. 2B is a plan view of a magnet and magnetic yokes of thetorque sensor of FIG. 1;

[0034]FIG. 2C is an elevation view of the magnet and the magnetic yokesof the torque sensor of FIG. 1;

[0035]FIG. 3A is a perspective view of a torque sensor according to amodification of the first embodiment;

[0036]FIG. 3B is an exploded perspective view of the torque sensor ofFIG. 3A;

[0037]FIG. 4A is a schematic view of the magnet and the magnetic yokeswhen a torsion bar is twisted in a direction according to the firstembodiment;

[0038]FIG. 4B is a schematic view of the magnet and the magnetic yokeswhen the torsion bar is not twisted according to the first embodiment;

[0039]FIG. 4C is a schematic view of the magnet and the magnetic yokeswhen the torsion bar is twisted in another direction according to thefirst embodiment;

[0040]FIG. 4D is a chart showing a relationship between magnetic fluxdensity and twist angle of the torsion bar according to the firstembodiment;

[0041]FIG. 5 is an exploded perspective view of a torque sensoraccording to a second embodiment of the present invention;

[0042]FIG. 6 is a cross sectional view of the torque sensor of FIG. 5;

[0043]FIG. 7 is an exploded perspective view of a part of a torquesensor according to a third embodiment of the present invention;

[0044]FIG. 8 is an exploded perspective view of a part of a torquesensor according to a fourth embodiment of the present invention;

[0045]FIG. 9 is an exploded perspective view of a part of a torquesensor according to a fifth embodiment of the present invention;

[0046]FIG. 10 is an exploded perspective view of a part of a torquesensor according to a sixth embodiment of the present invention;

[0047]FIG. 11 is a chart showing a relationship between magnetic fluxdensity and magnetic or mechanical angle of the torsion bar according toa seventh embodiment of the present invention;

[0048]FIG. 12 is a cross sectional view of a torque sensor according toan eighth embodiment of the present invention;

[0049]FIG. 13 is a cross sectional view of a torque sensor for a purposeof comparing with the torque sensor according to the eighth embodiment;

[0050]FIG. 14 is a plan view of a torque sensor according to a ninthembodiment of the present invention;

[0051]FIG. 15 is a cross sectional view of the torque sensor of FIG. 14;

[0052]FIG. 16 is a cross sectional view of a torque sensor according toa modification of the ninth embodiment;

[0053]FIG. 17 is a schematic view of an entire electric power steeringsystem according to a tenth embodiment of the present invention;

[0054]FIG. 18 is a cross sectional view of a torque sensor according toan eleventh embodiment of the present invention;

[0055]FIG. 19 is a cross sectional view of a torque sensor according toa twelfth embodiment of the present invention;

[0056]FIG. 20A is a perspective view of a torque sensor according to athirteenth embodiment of the present invention;

[0057]FIG. 20B is a plan view of a rotation transmission member of thetorque sensor of FIG. 20A;

[0058]FIG. 21A is a cross sectional view of a torque sensor mounted on acolumn housing according to a fourteenth embodiment of the presentinvention;

[0059]FIG. 21B is a schematic view of the torque sensor of 21A as viewedaxially;

[0060]FIG. 22A is a cross sectional view of a torque sensor mounted on acolumn housing according to a modification of the fourteenth embodiment;and

[0061]FIG. 22B is a schematic view of the torque sensor of FIG. 22A asviewed axially.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] (First Embodiment)

[0063] A torque sensor 1 according to a first embodiment is describedwith reference to FIGS. 1 to 4D.

[0064]FIG. 1 shows an exploded perspective view of the torque sensor 1.FIG. 2A is a cross sectional view of the torque sensor 1. FIGS. 2B and2C show plan and elevation views of a magnet and magnetic yokes,respectively.

[0065] The torque sensor 1 is applicable, for example, to an electricpower steering system for a vehicle (refer to FIG. 17) and disposedbetween an input shaft 2 and an output shaft 3 that constitute asteering shaft. The torque sensor 1 is for detecting steering torqueapplied to the steering shaft.

[0066] The torque sensor 1 is composed of a torsion bar 4 (resilientmember) connecting coaxially the input shaft 2 and the output shaft 3, amagnet 5 (ferromagnetic member) fixed to an axial end of the input shaft2 on a side of the output shaft 3, a pair of magnetic yokes 6 (softmagnetic member) fixed to an axial end of the output shaft 3 and amagnetic sensor 7 for detecting magnetic flux density generated betweenthe pair of the magnetic yokes 6.

[0067] Opposite axial ends of the torsion bar 4 are inserted into holesof the input and output shafts 2 and 3, respectively, and fixed via pins8 to the other axial end of the input shaft 2 and the other axial end ofthe output shaft 3, respectively. The torsion bar 4 has giventorsion/torque characteristics necessary for bringing an adequaterotating displacement of the axial end thereof relative to the otheraxial end thereof. Accordingly, when the torsion bar 4 is twisted, theaxial end of the input shaft 2 can be rotated, or circumferentiallyshifted, relative to the axial end of the output shaft 3.

[0068] The magnet 5, which is formed in a ring shape and composed of Nand S poles that are alternately arranged in a circumferential directionthereof, is positioned outside an outer circumference of the torsion bar4. The magnet 5 has, for example, 24 poles.

[0069] As shown in FIG. 1, each of the pair of magnetic yokes 6 (6A, 6B)is formed in a ring shape and arranged around and in a vicinity of anouter circumference of the magnet 5. Each of the magnetic yokes 6A or 6Bis provided with claws 6 a which are spaced circumferentially atconstant intervals and whose number is equal to that of N or S poles ofthe magnet 5 (12 pieces). The pair of the magnetic yokes 6 are fixed toand supported by a fixing base or holder 9 (refer to FIG. 2) in such amanner that the claws 6 a of the magnetic yoke 6A and the claws 6 a ofthe magnetic yoke 6B extend axially in a direction of coming close toeach other and are positioned to circumferentially alternate with oneanother.

[0070] In a state that the torsion bar 4 is not twisted (when torsiontorque is not applied to the torsion bar 4 to cause the input shaft 2 torotate relative to the output shaft 3), each axial center of the claws 6a of the magnetic yokes 6 (6A and 6B) is positioned to coincide with aboundary between immediately adjacent N and S poles of the magnet 5.

[0071] As more clearly shown in FIG. 2C, the magnetic sensor 3 ispositioned in an axial gap G existing between the magnetic yoke 6A andthe magnetic yoke 6B and detects the magnetic flux density generatedbetween the magnetic yokes 6A and 6B. The magnetic sensor 3 is fixed toa given position of a housing (not shown) without coming in contact withthe magnetic yokes 6.

[0072] The magnetic sensor 7 has a hole element, a hole IC or a magneticresistance element which outputs an electric signal (for example,voltage signal) whose value is converted from a value of the detectedmagnetic flux density.

[0073] Though the magnet 5 is fixed to the axial end of the input shaft2 and the magnetic yokes 6 are fixed to an axial end of the output shaft3 in the above embodiment, as shown in FIGS. 1 to 2C, the magnet 5 maybe fixed to the axial end of the torsion bar 4 on a side of the inputshaft 2 and the magnetic yokes 6 may be fixed to the other axial end ofthe torsion bar 4 on a side of the output shaft 3, as shown in FIGS. 3Aand 3B. In this case, an inner surface of the ring shaped magnet 5 ispress fitted to an outer surface of the torsion bar 4 and an inner holeof a holder 9A for supporting the ring shaped magnetic yokes 6A and 6Bis press fitted to the outer surface of the torsion bar 4.

[0074] The magnet 5 and the magnetic yokes 6 are assembled in the torquesensor 1 and the magnet 5 is positioned relative to the magnetic yokes 6through the following steps;

[0075] (1) fixing the ring shaped magnetic yokes 6 to the torsion bar 4on a side of the output shaft 3 or the axial end of the output shaft 3,for example, by press fitting or gluing,

[0076] (2) inserting the magnet 5 into the ring shaped magnetic yokes 6and holding the magnet 5 therein so as to allow a free rotation relativeto the torsion bar 4 or the input shaft 2 for tentative assembly,

[0077] (3) defining a magnet rest position where the magnet 5 restswithin the ring shaped magnet yokes 6 due to magnetic attracting forcegenerated between the magnet 5 and the ring shaped magnet yokes 6, and

[0078] (4) fixing the magnet 5 to the torsion bar 4 on a side of theinput shaft 2 or the axial end of the input shaft 2 in such a mannerthat the magnet 5 maintains the magnet rest position.

[0079] An operation of the torque sensor 1 is described hereinafter.

[0080] As shown in FIG. 4B, in the state that torque is not applied tothe torsion bar 4 so that the input shaft 2 is not rotated relative tothe output shaft 3, that is, at a neutral position that the torsion bar4 is not twisted, each axial center of the claws 6 a of the magneticyokes 6 coincides with a boundary between the immediately adjacent N andS poles of the magnet 5. In this case, since the number of lines ofmagnetic force passing between each of N poles and each of the claws 6 ais equal to that passing between the each of the claws 6 a and each of Spoles, the lines of magnetic force are closed within the respectivemagnetic yokes 6A and 6B and do not leak into the axial gap G betweenthe magnetic yoke 6A and the magnetic yoke 6B. Accordingly, the value ofthe magnetic flux density to be detected by the magnetic sensor 7 iszero, as shown in FIG. 4D.

[0081] As shown in FIG. 4A or 4C, in a state that torque is applied tothe torsion bar 4 so that the input shaft 2 is rotated relative to theoutput shaft 3, that is, when the torsion bar 4 is twisted, an angularposition of the magnet 5, which is fixed to the input shaft 2, relativeto the pair of magnetic yokes 6, which are fixed to the output shaft 3,is circumferentially changed. As the each axial center of the claws 6 aof the magnetic yokes 6 is circumferentially shifted from a boundarybetween the respective N and S poles of the magnet 5, the number oflines of magnetic force having N or S pole increases in each of themagnetic yokes 6A and 6B. Since polarity of the lines of magnetic forcewhose number increases in one of the magnetic yokes 6A is opposite tothat in the other of the magnetic yokes 6B, the magnetic flux density isgenerated between the magnetic yokes 6A and 6B, that is, in the axialgap G. The value of the magnetic flux density is about proportional to atwisted amount of the torsion bar 4 and its polarity can be invertedaccording to the direction in which the torsion bar 4 is twisted.

[0082] Advantages of the torque sensor 1 according to the firstembodiment are described hereinafter.

[0083] When the torsion bar 4 is twisted and the relative position ofthe magnet 5 to the pair of magnetic yokes 6 is circumferentiallychanged, the magnetic flux density between the pair of magnetic yokes 6is changed at an entire circumference thereof and the value of themagnetic flux density is identical at any circumferential position.Accordingly, if the magnetic sensor 7 is position at a given position inthe axial gap G with which the magnetic yokes 6A and 6B are opposed toeach other, the magnetic sensor 7 can detect the magnetic flux densitybetween the pair of magnetic yokes 6 without contacting the magneticyoke 6. Therefore, detecting reliability of the torque sensor 1 ishigher since the electric contacts (for example, the brush and the slipring) for the magnetic sensor 7 are not necessary.

[0084] Further, since the each axial center of the claws 6 a of themagnetic yokes 6 coincides with a boundary between the immediatelyadjacent N and S poles of the magnet 5, when the torsion bar 4 is nottwisted, a neutral point of the magnetic sensor 7 can be never shifted,even if magnetic force of the magnet 5 is changed due to temperaturechange, as shown in FIG. 4D. Accordingly, the torque sensor 1 isunlikely to be affected by offset drift and accuracy thereof in avicinity of the neutral point is more stable.

[0085] Furthermore, since, after the magnet rest position is defined,the magnet 5 is fixed to the torsion bar 4 or the input shaft 2, so asto maintain the magnet rest position, the position of the magnet 5relative to the magnetic yokes 6 can be accurately defined so that theoutput of the magnetic sensor is substantially zero, when the torsionbar 4 is not twisted.

[0086] (Second Embodiment)

[0087] A torque sensor 1 according to a second embodiment is describedwith reference to FIGS. 5 and 6.

[0088]FIG. 5 shows an exploded perspective view of a torque sensor 1.FIG. 6 is a cross sectional view of the torque sensor 1.

[0089] The torque sensor 1 according to the second embodiment has a pairof magnetic flux collective rings 10 (auxiliary soft magnetic member) inaddition to components of the first embodiment.

[0090] Each of the magnetic flux collective rings 10 (10A, 10B) is madeof same soft magnetic material as the magnetic yokes 6 and formed in aring shape. The magnetic flux collective rings 10A and 10B arepositioned around and in a vicinity of outer circumferences of themagnetic yokes 6A and 6B, respectively.

[0091] Each of the magnetic flux collective rings 10 is provided at acircumferential position thereof with a flat collective plate 10 a. Thecollective plates 10 a of the magnetic flux collective rings 10A and 10Bare axially opposed to each other. Axial distance between the collectiveplates 10 a is shorter than that between the other parts of the magneticflux collective rings 10A and 10B. The magnetic sensor 7 is positionedbetween the collective plates 10 a axially opposed to each other anddetects magnetic flux density generated between the collective plates 10a.

[0092] With the construction mentioned above, magnetic flux generatedfrom the magnet 5 is collected with priority on the collective plates 10a via the magnetic yokes 10, since the magnetic flux collective rings 10constitute a part of magnetic circuit. The magnetic sensor 7 detectsmagnetic flux density between the collective plates 10 a, whose value isan average value of the magnetic flux density between the entirecircumferences of the magnetic yokes 6. Accordingly, in the torquesensor 1 according to the second embodiment, detecting errors are hardlycaused by manufacturing or errors, assembly inaccuracy of the componentsconstituting the magnetic circuit or misalignment between the input andoutput shafts 2 and 3.

[0093] (Third Embodiment)

[0094] A torque sensor 1 according to a third embodiment is describedwith reference to FIG. 7. FIG. 7 shows an exploded perspective view of apart of the torque sensor 1.

[0095] The torque sensor 1 according to the third embodiment has twomagnetic sensors 2 which are positioned in the axial gap between themagnetic yokes 6A and 6B. Magnetism detecting directions of therespective magnetic sensors 7 are opposite to each other, as shown inarrows marks in FIG. 7. Each of the magnetic sensors 7 is connected to adifferential circuit 11. The differential circuit 11 outputs a torquesignal after output signals from the magnetic sensors 7, which are inputto the differential circuit 11, are processed differentially therein.

[0096] In case of a single magnetic sensor 7, the detection fluctuationdue to a position where the magnetic sensor is located is relativelylarge. However, as the torque sensor 1 according to the third embodimenthas two magnetic sensors 7, the detecting fluctuation due to positionswhere the magnetic sensors are located is smaller.

[0097] Further, output difference between the magnetic sensors 7 can beeffectively used for canceling temperature drift and increasingdetection sensitivity.

[0098] The differential circuit 11 may be or not be a component of thetorque sensor 1. Unless the differential circuit is the component of thetorque sensor 1, ECU (not shown) plays a roll of the differentialcircuit 11 and may execute differential processes based the output ofthe magnetic sensors 7 for calculating the torque.

[0099] The two magnetic sensors according to third embodiment may beapplied to the second embodiment, too.

[0100] (Fourth Embodiment)

[0101] A torque sensor 1 according to a fourth embodiment is describedwith reference to FIG. 8. FIG. 8 shows an exploded perspective view of apart of a torque sensor 1.

[0102] The torque sensor 1 according to the fourth embodiment has twomagnetic sensors 7, which is similar to the third embodiment. The twomagnetic sensors 7 are arranged symmetrically with respect to thetorsion bar 4 (on radially opposite sides of the torsion bar 4) in theaxial gap between the magnetic yokes 6A and 6B. Magnetism detectingdirections of the respective magnetic sensors 7 are opposite to eachother, as shown in arrows marks in FIG. 8. Each of the magnetic sensors7 is connected to a differential circuit 11. The differential circuit 11outputs a torque signal after output signals from the magnetic sensors7, which are input to the differential circuit 11, are processeddifferentially therein.

[0103] As the torque sensor 1 according to the fourth embodiment has twomagnetic sensors 7, which is similar to the third embodiment, thedetection is less affected by positions where the magnetic sensors arelocated and, therefore, the detection accuracy is higher, compared withthat of the single magnetic sensor.

[0104] Further, output difference between the magnetic sensors 7 can beeffectively used for canceling temperature drift and increasingdetection sensitivity twice because detection physical quantity isdoubled. Moreover, the misalignment between the input and output shaft 2and 3 is less affected on detecting accuracy.

[0105] The differential circuit 11 may be or not be a component of thetorque sensor 1. Unless the differential circuit 11 is the component ofthe torque sensor 1, ECU (not shown) plays a roll of the differentialcircuit 11 and may execute differential processes based the outputs ofthe magnetic sensors 7 for calculating the torque.

[0106] (Fifth Embodiment)

[0107] A torque sensor 1 according to a fifth embodiment is describedwith reference to FIG. 9. FIG. 9 shows an exploded perspective view of apart of a torque sensor 1.

[0108] The torque sensor 1 according to the fifth embodiment has twomagnetic sensors 7 arranged symmetrically with respect to the torsionbar 4 in the collective plates 10 a of the magnetic flux collectiverings 10 (10A, 10B), which is similar to the second embodiment. Thecollective plates 10 a according to the fifth embodiment are two pairsof collective plates 10 a which are circumferentially spaced at 180°intervals, as shown in FIG. 9.

[0109] Each of the two magnetic sensors 7 is positioned between one ofthe pairs of the collective plates 10 a axially opposed to each other.Magnetism detecting directions of the respective magnetic sensors 7 areopposite to each other, as shown in arrows marks in FIG. 9. Each of themagnetic sensors 7 is connected to a differential circuit 11. Thedifferential circuit 11 outputs a torque signal after output signalsfrom the magnetic sensors 7 are processed differentially therein.

[0110] The fifth embodiment has not only an advantage that each of themagnetic sensors 7 detects an average value of the magnetic flux densitybetween the entire circumferences of the magnetic yokes 6 because ofusing the magnetic flux collective rings 10 but also another advantagethat the detection sensitivity is twice and the misalignment between theinput and output shaft 2 and 3 is less affected on detecting accuracy.

[0111] (Sixth Embodiment)

[0112] A torque sensor 1 according to a sixth embodiment is describedwith reference to FIG. 10. FIG. 10 shows an exploded perspective view ofa part of a torque sensor 1.

[0113] The torque sensor 1 according to the sixth embodiment has morethan two pieces of magnetic sensors 7 (three pieces of magnetic sensors7 in this embodiment).

[0114] The three magnetic sensors 7, which are circumferentially spacedat constant intervals, are arranged in the axial space between themagnetic yokes 6A and 6B and connected to a calculation circuit 12.Magnetism detecting directions of the respective magnetic sensors 7 aresame to one another. The calculation circuit 12 outputs a torque signalafter processing to add or average outputs of the three magnetic sensors7.

[0115] Since the torque sensor 1 according to the sixth embodiment hasthree magnetic sensors 7 and the outputs thereof are processed throughadding or average calculation, the detecting accuracy is remarkablyimproved, compared with that of the single magnetic sensor 7, whosedetection of the magnetic flux density is largely affected by theposition where the magnetic sensor 7 is located.

[0116] The calculation circuit 12 may be or not be a component of thetorque sensor 1. Unless the calculation circuit 12 is the component ofthe torque sensor 1, ECU (not shown) plays a roll of the calculationcircuit 12 and may execute adding or average processes based the outputsof the magnetic sensors 7 for calculating the torque.

[0117] (Seventh Embodiment)

[0118]FIG. 11 shows a graph illustrating a relationship between a twistangle of the torsion bar 4(a displacement angle of the magnet 5 to themagnetic yokes 6) and magnetic flux density generated between themagnetic yokes 6. The twist angle of the torsion bar 4 is shown as amaximum twist angle of the torsion bar 4 in relation with a polar numberof the magnet 5 or the magnetic yokes 6.

[0119] As shown in FIG. 11, if the following formula (1) is satisfied,torque can be accurately detected (sensible zone).

θ_(max) ×n≦120[deg]  (1)

[0120] where θ_(max) is a maximum twist angle of the torsion bar 4 and nis a polar number of the magnet 5 or the magnetic yokes 6.

[0121] Preferably, if the following formula (2) is satisfied, torque canbe more accurately detected since the value of the magnetic flux densityis more linearly changed with respect to the maximum twist angle of thetorsion bar 4 (linear zone).

θ_(max) ×n≦60 [deg]  (2)

[0122] (Eighth Embodiment)

[0123] A torque sensor 1 according to an eighth embodiment is describedwith reference to FIG. 12. FIG. 12 shows an exploded perspective view ofa torque sensor 1.

[0124] The torque sensor 1 according to the eighth embodiment has amagnet 5 whose axial length is longer than that of the magnetic yokes 6,as shown in FIG. 12.

[0125] For example, in the torque sensor 1 in which an axial length ofthe magnet 5 is substantially equal to or shorter than that of themagnetic yokes 7, a radial gap between the magnet 5 and the magneticyokes 7 is prone to be filled with iron filings Q, if invaded into thetorque sensor 1 from outside, which cause short circuit of the magneticcircuit and, thus, erroneous detection.

[0126] However, in a case that opposite axial ends of the magnet 5axially protrude out of the opposite axial ends of the magnetic yokes 7,as shown in the eighth embodiment, the iron filings Q are stuck to edgesof the magnet 5 (since the magnet 5 has characteristics that magneticflux is concentrated to the edges thereof), which does not adverselyaffect on the magnetic circuit for detecting the torque so that theerroneous detection may be avoided.

[0127] (Ninth Embodiment)

[0128]FIG. 14 shows a plan view of a torque sensor 1 according to aninth embodiment. FIG. 15 shows a cross sectional view of the torquesensor 1 according to the ninth embodiment.

[0129] The torque sensor 1 according to the ninth embodiment has amagnetic seal 13 (magnetic material) covering a substantially entireportion of the magnetic circuit thereof.

[0130] The magnetic seal 13 is formed in cylindrical shape, as shown inFIGS. 14 and 15. The magnetic seal 13 serves to shut out influences ofterrestrial magnetism and magnetic fields generated around the torquesensor 1 so that the erroneous detection is avoided.

[0131] Further, as shown in FIG. 16, the magnetic seal 13 may cover onlythe magnetic sensor 7 without covering the entire portion of themagnetic circuit of the torque sensor 1.

[0132] (Tenth Embodiment)

[0133] An electric power steering system incorporating the torque sensorof the present invention according to a tenth embodiment is describedwith FIG. 17.

[0134] The electric power steering system according to the tenthembodiment is composed of an electric motor 14 for giving additionalforce to a steering power transmission mechanism 14A, which connect theoutput shaft 3 and wheels 14B, for assisting the steering torque appliedto a steering 14 by a driver, the torque sensor 1 for detecting thesteering torque applied to the steering 14 and a control circuit forcontrolling current to be supplied to the electric motor 15 in responseto the value of the torque detected by the torque sensor 1. Theconstruction of the torque sensor 1 is, for example, same as that of thefirst embodiment.

[0135] As the electric power steering system mentioned above does nothave a coil for detecting change of magnetic fields and a coil forcompensating temperature change, which are provided in a conventionalelectric power steering system, a large housing for accommodating thesecoils is not necessary.

[0136] Further, torque sensor 1 does not emit electric noises with lesspower consumption, since alternating current is not applied to the coilas in the conventional torque sensor.

[0137] The magnetic sensor 7 uses the hole IC which causes the torquesensor 1 compact and inexpensive since auxiliary circuits such as a gainadjusting circuit, an offset adjusting circuit and a temperaturecompensation circuit are not necessary, so the torque sensor 1 can becomposed of less number of components.

[0138] Further, since the hole IC does not require an oscillatingcircuit so that noises are hardly radiated, the hole IC does not give anoise problem to surrounding electric devices.

[0139] Furthermore, since electric components other than the hole IC arenot necessary, the magnet sensor 7 can be operative with less powerconsumption and at relatively high temperature, which the hole IC canendure for its use.

[0140] Moreover, since the gain adjustment, the offset adjustment andthe temperature compensation, which have been executed by ECU in theconventional torque sensor, can be performed within the hole IC, qualityassurance of the torque sensor 1 is available as a single body and, ifthe torque sensor 1 fails, only the failed torque sensor 1 can bereplaced without consulting with the other components such as ECU.Further, it is not necessary to initialize the torque sensor 1, when thetorque sensor 1 is assembled to a torque sensor system, for example, tothe electric power steering system, resulting higher productivity andlower cost.

[0141] (Eleventh Embodiment)

[0142] As shown in FIG. 18, an electric power steering system accordingto an eleventh embodiment has a circuit board 17 in which the controlcircuit 16(refer to FIG. 17) and the magnetic sensor 7 for the torquesensor 1 are simultaneously installed. The circuit board 17 is fixed,for example, with screws to a housing 18 in which the torque sensor 1 isaccommodated.

[0143] In this case, wire harnesses and connectors for connecting thetorque sensor 1 and the control circuit 16 are not necessary, whichresults in cost saving and better reliability because of no electriccontacts.

[0144] (Twelfth Embodiment)

[0145] As shown in FIG. 19, in an electric power steering systemaccording to a twelfth embodiment, the magnetic sensor 7 is mounted on aconnector or plug 21 of a wire harnesses 20 for connecting the torquesensor 1 and the control circuit 16.

[0146] In this case, if the connector 21 with the magnet sensor 7 issimply inserted into a housing 18 of the torque sensor 1, the assemblyof the magnet sensor 7 is simpler.

[0147] (Thirteenth Embodiment)

[0148] In an electric power steering system according to a thirteenthembodiment, a sensing portion S of the torque sensor 1 can be assembledat a later time. The sensing portion S is composed of a ring shapedmagnet 5, a pair of ring shaped magnetic yokes 6(6A, 6B) and a magneticsensor 7.

[0149] As shown in FIG. 20, an input shaft 3, a torsion bar 4 and anoutput shaft 3 are axially in alignment with one another. The sensingportion S is positioned axially in parallel with the torsion bar 4. Theinput shaft 2 is connected to the magnet 5 via a first torquetransmission member that is composed of a gear 22 attached coaxially tothe input shaft 2 and a gear 23 attached coaxially to the magnet 5 a.The gears 22 and 23 are in mesh so that rotation of the input shaft 2 istransmitted to the magnet 5. The output shaft 3 is connected to themagnetic yokes 6 via a second torque transmission member that iscomposed of a gear 24 attached coaxially to the output shaft 3 and agear 25 attached coaxially to the magnetic yokes 6. The gears 24 and 25are in mesh so that rotation of the output shaft 3 is transmitted to themagnetic yokes 6.

[0150] With the construction mentioned above, the sensing portion S canbe assembled separately after the input shaft 2 with the gear 22, thetorsion bar 4, the output shaft 3 with the gear 25, the electric motor15 (refer to FIG. 17) and the steering power transmission mechanism 14A(refer to FIG. 17) are assembled in advance. Accordingly, it is simplerto assemble the sensing portion S to the electric power steering system.Further, the sensing portion S can be replaced as a single body, whichfacilitates maintenance operation.

[0151] (Fourteenth Embodiment)

[0152] As shown in FIG. 21, an electric power steering system accordingto a fourteenth embodiment has a magnetic seal 26 surrounding the torquesensor 1. The magnetic seal 26 covers entire outer circumference of acolumn housing 27 (for example, made of aluminum) in which the torquesensor 1 is housed.

[0153] The torque sensor 1 to be used in the electric power steeringsystem is prone to erroneously detect the torque, if influenced byoutside magnetic fields generated by, for example, onboard speakers(incorporating magnet members). Accordingly, magnetic sealing around theouter circumference of the torque sensor 1 prevents the torque sensor 1from erroneously detecting due to the outside magnetic fields.

[0154] As shown in FIG. 22, in place of magnetic sealing the entireouter circumference of the column housing 27, only a portion of thecolumn housing 27 where the magnetic sensor 7 is positioned may bemagnetic sealed.

[0155] In the embodiments mentioned above, instead that the magnet 5 isconnected to the first shaft 2 or the torsion bar 4 on a side of thefirst shaft 2 and the magnetic yoke 6 is connected to the second shaft 3or the torsion bar 4 on a side of the second shaft 3, the magnet 5 maybe connected to the second shaft 3 or the torsion bar 4 on a side of thesecond shaft 3 and the magnetic yokes 6 may be connected to the firstshaft 2 or the torsion bar 4 on a side of the first shaft 2.

[0156] Further, to assemble the magnet 5 and the magnetic yokes 6 in thetorque sensor 1, after the magnet 5 is fixed at first to the torsion bar4 or one of the input and output shafts 2 and 3, the magnetic yokes 6may cover the magnet 5 to define magnetic yoke rest position and, then,the magnetic yokes 6 may be fixed to the torsion bar 4 or the other ofthe input and output shafts 2 and 3 to maintain the magnetic yokeposition.

What is claimed is:
 1. A torque sensor for detecting torsion torque tobe applied to a first shaft and a second shaft comprising: a resilientmember disposed between and fixed to the first and second shafts so thatthe first shaft, the resilient member and the second shaft are coaxiallyin alignment with one another, the resilient member being resilientlytwisted, when torsion torque is applied to the first shaft and thesecond shaft; a ferromagnetic member connected to one of a givenposition of the first shaft and a given position of the resilient memberon a side of the first shaft and rotatable together therewith, theferromagnetic member producing a magnetic field; a soft magnetic memberconnected to one of a given position of the second shaft and anothergiven position of the resilient member on a side of the second shaft androtatable together therewith, the soft magnetic member being positionedwithin the magnetic field and forming a magnetic circuit so thatmagnetic flux density generated in the magnetic circuit is varied whenrotation displacement of the soft magnetic member relative to theferromagnetic member is changed according to the twist of the resilientmember; and a magnetic sensor positioned in a vicinity of and withoutcontacting the soft magnetic member for detecting the magnetic fluxdensity generated in the magnetic circuit. 2.A torque sensor accordingto claim 1, further comprising: an auxiliary soft magnetic member havinga magnetic flux collective portion, the auxiliary soft magnetic memberbeing positioned in a vicinity of the soft magnetic member forintroducing magnetic flux from the soft magnetic member andconcentrating the same to the magnetic flux collective portion, whereinthe magnetic sensor detects the magnetic flux density generated in themagnetic circuit through the magnetic flux collective portion.
 3. Atorque sensor according to claim 1, wherein the ferromagnetic member isa ring shaped magnet having N and S poles alternately arrangedcircumferentially, the soft magnetic member is a pair of ring shapedmagnetic yokes that are positioned around an outer circumference of themagnet and axially opposed to each other with an axial gap therebetween,each of the magnetic yokes having claws which are radially spaced atconstant intervals and whose number is equal to that of each of the Nand S poles and the claws of one of the magnetic yokes axially extendingtoward and being positioned to alternate circumferentially with those ofthe other of the magnetic yoke, and the magnetic sensor is positioned inthe axial gap between the pair of the magnetic yokes.
 4. A torque sensoraccording to claim 2, wherein the ferromagnetic member is a ring shapedmagnet having N and S poles alternately arranged circumferentially, thesoft magnetic member is a pair of ring shaped magnetic yokes that arepositioned around an outer circumference of the magnet and axiallyopposed to each other with an axial gap therebetween, each of themagnetic yokes having claws which are radially spaced at constantintervals and whose number is equal to that of each of the N and S polesand the claws of one of the magnetic yokes axially extending toward andbeing positioned to alternate circumferentially with those of the otherof the magnetic yoke, The auxiliary soft magnetic member is a pair ofring shaped auxiliary magnetic yokes each having the magnetic fluxcollective portion, one of the auxiliary magnetic yoke being positionedaround an outer circumference of the one of the magnetic yoke and theother of the auxiliary magnetic yoke being positioned around an outercircumference of the other of the magnetic yoke so that the magneticflux collective portions of the pair of the auxiliary magnetic yokes areaxially opposed to each other with an axial gap therebetween, and themagnetic sensor is positioned in the axial gap between the magnetic fluxcollective portions.
 5. A torque sensor according to claim 4, wherein alength of the axial gap between both the magnetic flux collectiveportions is shorter than that between both portions of the pair ofauxiliary magnetic yokes other than the magnetic flux collectiveportions.
 6. A torque sensor according to claim 3, further comprising:first rotation transmitting member through which the magnet is connectedto the one of the given position of the first shaft and the givenposition of the resilient member on a side of the first shaft; andsecond rotation transmitting member through which the soft magneticmember is connected to the other of the given position of the secondshaft and the given position of the resilient member on a side of thesecond shaft, wherein the magnet and the pair of magnetic yokes arepositioned axially in parallel with the resilient member.
 7. A torquesensor according to claim 6, wherein the first rotation transmittingmember is a first gear fixed to the first shaft and a second gear fixedto the magnet, the first and second gears being in mesh with each other,and the second rotation transmitting member is a third gear fixed to thesecond shaft and a fourth gear fixed to the magnetic yokes, the thirdand fourth gears are being in mesh with each other.
 8. A torque sensoraccording to any one of claim 3 to 7, wherein each axial center of theclaws of both the magnetic yokes is positioned to substantially coincidewith a boundary between immediately adjacent N and S poles of themagnet, when a twist angle of the resilient member shows a referencevalue.
 9. A torque sensor according to claim 1, wherein the magnetsensor is two sensors which are positioned apart and whose magnetismdetecting directions are opposite to each other.
 10. A torque sensoraccording to claim 9, wherein the two sensors are positionedsymmetrically with respect to an axis of the soft magnetic member.
 11. Atorque sensor according to claim 1, wherein the magnet sensor is morethan two sensors which are positioned circumferentially at constantintervals and whose magnetism detecting directions are same to oneanother.
 12. A torque sensor according to claim 4, wherein each of theauxiliary magnetic yokes has a pair of the collective portionscircumferentially spaced at 180° intervals and the pair of thecollective portions of one of the auxiliary magnetic yoke are axially inalignment with the pair of the collective portions of the other thereof,respectively, and the magnet sensor has two sensors each of which ispositioned between the collective portions of the auxiliary magneticyokes axially aligned with each other and whose magnetism detectingdirections are opposite to each other.
 13. A torque sensor according toclaim 4, wherein each of the auxiliary magnetic yokes has a plurality ofthe collective portions circumferentially spaced at constant intervalsand the plurality of the collective portions of one of the auxiliarymagnetic yoke are axially in alignment with the plurality of thecollective portions of the other thereof, respectively, and the magnetsensor has a plurality of sensors each of which is positioned betweenthe collective portions of the auxiliary magnetic yokes axially alignedwith each other and whose magnetism detecting directions are same to oneanother.
 14. A torque sensor according to claim 3, wherein the followingformula is satisfied, θ_(max) ×n≦120[deg] where θ_(max) is a maximumtwist angle of the resilient member and n is a polar number of each ofthe magnet and the magnetic yokes.
 15. A torque sensor according toclaim 3, wherein the following formula is satisfied, θ_(max) ×n≦60[deg]where θ_(max) is a maximum twist angle of the resilient member and n isa polar number of each of the magnet and the magnetic yokes.
 16. Atorque sensor according to claim 1, further comprising: a magnetic sealcovering at least outer circumference of the magnetic sensor.
 17. Atorque sensor according to claim 3, wherein axial length of the magnetis longer than that of the magnetic yoke.
 18. An electric power steeringsystem for steering a vehicle wheel incorporating the torque sensoraccording to claim 1, comprising: a steering to which steering torque isapplied, one end of the steering is connected to one of the first andsecond shafts; a steering power transmission mechanism whose one end isconnected to the wheel and whose the other end is connected to the otherof the first and second shafts; an electric motor connected to thesteering power transmission mechanism and in circuit with the magneticsensor of the torque sensor; and a control circuit for generatingcontrol current to be supplied to the electric motor in response to adetected output of the magnetic sensor, wherein the electric motor givesa drive force to the steering power transmission mechanism in responseto the control current for assisting the steering torque applied to thesteering.
 19. An electric power steering system according to claim 18,wherein the magnetic sensor is hole IC.
 20. An electric power steeringsystem according to claim 18, wherein the control circuit has a plateboard on which the magnetic sensor is simultaneously mounted.
 21. Anelectric power steering system according to claim 18, furthercomprising: a connector of a wire harness connecting the control circuitand the torque sensor, wherein the magnetic sensor is mounted on theconnector.
 22. An electric power steering system according to claim 18,further comprising: a column housing in which the torque sensor ishoused; and a magnetic seal covering at least an outer circumference ofthe column housing where the magnetic sensor is positioned.
 23. Anelectric power steering system according to claim 22, wherein themagnetic seal covers only a part of the outer circumference in avicinity of the magnetic sensor.
 24. A method of assembling a torquesensor according to claim 1, comprising steps of: fixing the softmagnetic member to the one of the second shaft and the resilient memberon a side of the second shaft; tentatively assembling the ferromagneticmember to the soft magnetic member so as to allow a free rotationrelative to one of the first shaft and the resilient member on a side ofthe first shaft; defining a rest position where the ferromagnetic memberrests relative to the soft magnetic member due to magnetic attractingforce generated between the ferromagnetic member and the soft magneticmember; and fixing the ferromagnetic member to the one of the firstshaft and the resilient member on a side of the first shaft.
 25. Amethod of assembling a torque sensor according to claim 1, comprisingsteps of: fixing the ferromagnetic member to the one of the first shaftand the resilient member on a side of the first shaft; tentativelyassembling the soft magnetic member to the ferromagnetic member so as toallow a free rotation relative to one of the second shaft and theresilient member on a side of the second shaft; defining a rest positionwhere the soft magnetic member rests relative to the ferromagneticmember due to magnetic attracting force generated between theferromagnetic member and the soft magnetic member; and fixing the softmagnetic member to the one of the second shaft and the resilient memberon a side of the second shaft.